This invention relates to the thermo-transfer processes used to make reproductions from image-containing originals by methods which effect the transfer of images directly onto copy sheets or which alternatively provide master duplicating sheets usable in solvent duplicating processes. More specifically, however, this invention relates to means for improving image resolution in various thermo-transfer processes.
In general, thermo-transfer processes are used in making both facsimile copies, i.e. single copies prepared directly from and to the size of an image-containing original, and duplication master sheets for later use in solvent, duplicating processes. A complete description of such thermo-transfer processes, the materials used therein and compositions of the thermal transfer layers can be found in U.S. Pat. Nos. 3,122,997 and 3,122,998 the entire disclosures of which are hereby incorporated by reference. In general, however, the thermo-transfer processes useful in preparing either facsimile copies or master sheets employ an original, carrying infrared radiation absorbing characters and designs, and a waxy dye-containing transfer material capable of responding to heat patterns generated on the original which correspond to the characters and designs imprinted thereon.
When infrared radiation absorbing characters carried on an original sheet are subjected to the controlled exposure of infrared radiation, a differential heat pattern is generated in the sheet so that the areas in the sheet corresponding to the infrared radiation absorbing characters are elevated to high temperatures relative to the rather lower temperatures achieved in the non-imaged areas of the sheet. Thus, by means of controlled exposure, an image-containing original sheet can be made to develop a dominant heat pattern corresponding identically to the distinctive infrared radiation absorption pattern on the imaged original sheet.
After generation of a differential heat pattern on the imaged original sheet, the next step in the thermo-transfer process is to transfer the heat pattern produced from the original by conduction to a suitable transfer material coated on a suitable base. More specifically, transfer of the heat pattern is desirably accomplished by placing the original sheet in conductive relation with the transfer material during the interval in which the original sheet is exposed to infrared radiation. After exposure, the differential heat pattern generated on the original is conducted through any intervening layer to the transfer material.
When direct copies of the original are desired, a copy sheet is imtimately contacted with the transfer material. Then a heat pattern is generated on the original sheet and transferred to the waxy transfer material so that selective portions of the transfer material bond to the copy sheet. Production of the copy is thereafter accomplished by separating the non-adhered transfer material on the copy sheet thereby effecting an exact duplication on the copy sheet of the imaged characters or designs carried by the original.
It has been observed that lateral thermal diffusion in the waxy transfer layer degrades image resolution. One reason for this observed resolution degradation is the isotropic character of the dye and wax-containing transfer processes. In short, the dye-wax materials are equally thermally conductive in all directions. Thus, a differential heat pattern generated on the original and transferred to the dye-wax material has a tendency to spread laterally so as to cause blurred images on the final copy or master sheets.